If we’re going to pass immigration legislation, it is essential that we improve the enforcement of existing immigration laws, as Ramesh Ponnuru recently argued in Bloomberg View. Any proposal that does not meaningfully strengthen enforcement is a non-starter.

One persistent problem is that while many people believe that immigration enforcement is primarily about border enforcement, it is actually interior enforcement that matters most. Even with the most aggressive border enforcement, it is extremely unlikely that we can deter all border crossers at an acceptable cost. We can certainly reduce the number of border crossers, and we appear to have done so in recent years. A 2013 Council on Foreign Relations report (Managing Illegal Immigration to the United States) found that the number illegal crossings has fallen sharply over the past decade, yet it’s not clear how much of this decrease can be attributed to more effective enforcement as opposed to changing labor market conditions in the United States and in source countries. The report estimates that the apprehension rate along the U.S.-Mexico border is in the range of 40 to 55 percent. But apprehensions offer only part of the picture, as Matt Graham of the Bipartisan Policy Center has observed. Border Patrol officials estimate apprehensions, turn backs, and “got aways” (those who evade capture), and they divide the number of apprehensions and turn backs by the total number of illegal entries to determine the “effectiveness rate” of border enforcement. And it turns out that the effectiveness rate is quite high, having increased from 69.11 percent in 2006 to 83.98 percent in 2011. The problem, however, is that even if the effectiveness of border enforcement reaches 100 percent, we will still have a large unauthorized population already residing in the country and a continuing influx of people who enter the country legally only to overstay their visas.

Though estimates vary, a large share of unauthorized immigrants in the United States are visa overstayers rather than border crossers. In 2006, the Pew Hispanic Center offered a tentative estimate of the size of visa overstayer population. Drawing on the 1997 findings of Robert Warren, a demographer who had served for many years with what was then the U.S. Immigration and Naturalization Service and supplementing these findings with new work on the size of the unauthorized population, the Center estimated visa overstayers represent somewhere between 33 and 50 percent of the unauthorized population.

Interior enforcement proposals often center on E-Verify, an Internet-based system designed to check information provided on an employee’s Form I-9, Employment Eligibility Verification, against federal databases maintained by the Social Security Administration and the U.S. Department of Homeland Security to determine if she or he is authorized to work in the United States. Making E-Verify mandatory and penalizing employers who hire employees who fail to pass an E-Verify check would, in theory, greatly improve interior enforcement. The idea is that unauthorized immigrants will shift from employment in the formal sector to riskier, and often less lucrative, employment in the informal sector, and over time, the “magnet” of (relatively) lucrative employment opportunities will grow weaker, thus deterring would-be illegal border crossers and visa overstayers in the first place.

Yet there are a number of potential problems with mandatory E-Verify. The American Civil Liberties Union, which favors far more permissive immigration policies, warns that the federal databases on which the E-Verify system depends are rife with errors; they anticipate that it might exacerbate discrimination against lawful immigrants; and they emphasize its potential costs, which will include the cost of administering the expanded system and the lost tax revenue as unauthorized immigrants shift into the untaxed black economy. (It is true that heightened immigration enforcement has led employers to rely more heavily on staffing organizations, in part so that these firms can bear the risks associated with hiring unauthorized immigrants.) Henrik Temp of AEI summarized similar objections raised by the Competitive Enterprise Institute. More broadly, even if E-Verify works well, it will generate a large number of false positives that will cause considerable disruptions in a large and complex labor market like ours. So while the basic E-Verify concept has much to recommend it, mandatory E-Verify is definitely not a panacea.

There is, however, another, more promising approach for interior enforcement. In testimony to the House Judiciary Committee last fall, Janice Kephart, a fellow at the Center for Immigration Studies, a think tank that favors immigration restriction, has argued that the U.S. can implement an affordable and effective biometric entry-exit system in the near term. Assuming border enforcement gets more effective, a higher minimum wage goes into effect (whether we like it or not), and Kephart is right to suggest that a less porous entry-exit system is feasible, we’d go a long way towards reducing the size of the illegal influx. And though I’m extremely skeptical of most current guest-worker proposals, a Canadian-style guest-worker program might be another means of addressing the underlying demand for less-skilled labor in a manageable, coherent fashion, further reducing the influx.

Call me crazy, but it seems like immigration enforcement is something we can actually get right. So when conservative lawmakers insist that we do so before we consider blanket legalization for unauthorized immigrants who arrived in the U.S. as adults, they’re being entirely reasonable. Ponnuru’s immigration compromise strikes me as exactly right:

A better idea would have four parts: We’d increase enforcement of immigration laws at the border and in the workplace. We’d put people who were brought here illegally while they were minors but have otherwise obeyed the law on a path to full citizenship. We’d signal that amnesty for other illegal immigrants might be possible in the future once we’re sure that enforcement is working. And we’d reform our legal immigration policies to let in more high-wage workers.

This is a proposal that conservatives can responsibly get behind, and that can allow the congressional GOP to move on to higher priorities, like the long-term unemployment crisis.